For many years it has been an aim of scientists in the field of specifically targeted drug therapy to use monoclonal antibodies (MAbs) for the specific delivery of toxic agents to human cancers. Conjugates of tumor-associated MAbs and suitable toxic agents have been developed, but have had mixed success in the therapy of cancer, and virtually no application in other diseases, such as infectious and autoimmune diseases. The toxic agent is most commonly a chemotherapeutic drug, although particle-emitting radionuclides, or bacterial or plant toxins have also been conjugated to MAbs, especially for the therapy of cancer (Sharkey and Goldenberg, C A Cancer J Clin. 2006 July-August; 56(4):226-243) and, more recently, with radioimmunoconjugates for the preclinical therapy of certain infectious diseases (Dadachova and Casadevall, Q J Nucl Med Mol Imaging 2006; 50(3):193-204; incorporated herein by reference).
The advantages of using MAb-chemotherapeutic drug conjugates are that (a) the chemotherapeutic drug itself is structurally well defined; (b) the chemotherapeutic drug is linked to the MAb protein using very well defined conjugation chemistries, often at specific sites remote from the MAbs antigen binding regions; (c) MAb-chemotherapeutic drug conjugates can be made more reproducibly than chemical conjugates involving MAbs and bacterial or plant toxins, and as such are more amenable to commercial development and regulatory approval; and (d) the MAb-chemotherapeutic drug conjugates are orders of magnitude less toxic systemically than radionuclide MAb conjugates.
Early work on protein-drug conjugates indicated that a drug preferably is released in its original form, once it has been internalized into a target cell, for the protein-chemotherapeutic drug conjugate to be a useful therapeutic. Trouet et al. (Proc. Natl. Acad. Sci. USA 79:626-629 (1982)) showed the advantage of using specific peptide linkers, between the drug and the targeting moiety, which are cleaved lysosomally to liberate the intact drug. Notably, MAb-chemotherapeutic drug conjugates prepared using mild acid-cleavable linkers, such as those containing a hydrazone, were developed, based on the observation that the pH inside tumors was often lower than normal physiological pH (Willner et al., U.S. Pat. No. 5,708,146; Trail et al. (Science 261:212-215 (1993)). The first approved MAb-drug conjugate, Gemtuzumab Ozogamicin, incorporates a similar acid-labile hydrazone bond between an anti-CD33 antibody, humanized P67.6, and a potent calicheamicin derivative. Sievers et al., J Clin Oncol. 19:3244-3254 (2001); Hamann et al., Bioconjugate Chem. 13: 47-58 (2002). In some cases, the MAb-chemotherapeutic drug conjugates were made with reductively labile hindered disulfide bonds between the chemotherapeutic drugs and the MAb (Liu et al., Proc Natl Acad Sci USA 93: 8618-8623 (1996)).
Yet another cleavable linker involves cathepsin B-labile dipeptide spacers, such as Phe-Lys or Val-Cit, similar to the lysosomally labile peptide spacers of Trouet et al. containing from one to four amino acids, which additionally incorporated a collapsible spacer between the drug and the dipeptide (Dubowchik, et al., Bioconjugate Chem. 13:855-869 (2002); Firestone et al., U.S. Pat. No. 6,214,345 B1; Doronina et al., Nat Biotechnol. 21: 778-784 (2003)). The latter approaches were also utilized in the preparation of an immunoconjugate of camptothecin (Walker et al., Bioorg Med Chem Lett. 12:217-219 (2002)). Another cleavable moiety that has been explored is an ester linkage incorporated into the linker between the antibody and the chemotherapeutic drug. Gillimard and Saragovi have found that when an ester of paclitaxel was conjugated to anti-rat p75 MAb, MC192, or anti-human TrkA MAb, 5C3, the conjugate was found to exhibit target-specific toxicity. Gillimard and Saragovi, Cancer Res. 61:694-699 (2001).
The conjugates of the instant invention possess greater efficacy, in many cases, than unconjugated or “naked” antibodies or antibody fragments, although such unconjugated targeting molecules have been of use in specific situations. In cancer, for example, naked antibodies have come to play a role in the treatment of lymphomas (CAMPATH® (alemtuzumab) and RITUXAN® (rituximab)), colorectal and other cancers (ERBITUX® (cetuxumab) and AVASTIN® (bevacizumab)), breast cancer (HERECEPTIN® (trastuzumab)), as well as a large number now in clinical development (e.g., epratuzumab). In most of these cases, clinical use has involved combining these naked, or unconjugated, antibodies with other therapies, such as chemotherapy or radiation therapy.
A variety of antibodies are also in use for the treatment of autoimmune and other immune dysregulatory diseases, such as tumor necrosis factor (TNF) and B-cell (RITUXAN® (rituximab)) antibodies in arthritis, and are being investigated in other such diseases, such as the B-cell antibodies, RITUXAN® (rituximab) and epratuzumab, in systemic lupus erythematosus and Sjögren's syndrome, as well as juvenile diabetes and multiple sclerosis. Naked antibodies are also being studied in sepsis and septic shock, Alzheimer's disease, and infectious diseases. The development of anti-infective monoclonal antibodies has been reviewed recently by Reichert and Dewitz (Nat Rev Drug Discovery 2006; 5:191-195), incorporated herein by reference, which summarizes the priority pathogens against which naked antibody therapy has been pursued, resulting in only 2 pathogens against which antibodies are either in Phase III clinical trials or are being marketed (respiratory syncytial virus and methicillin-resistant Staphylococcus aureus), with 25 others in clinical studies and 20 discontinued during clinical study.
There is a need to develop more potent anti-pathogen or anti-cancer antibodies and other binding moieties. Such antibody-mediated therapeutics can be developed for the treatment of many different pathogens, including bacteria, fungi, viruses, and parasites, either as naked (unconjugated), radiolabeled, or drug/toxin conjugates. There is a further need to develop more effective antibody conjugates with intracellularly cleavable linkers, of use for the treatment of cancer, pathogens and other diseases. In the case of delivering drug/toxin or radionuclide conjugates, this can be accomplished by direct antibody conjugation or by indirect methods, referred to as pretargeting, where a bispecific antibody is used to target to the lesion, while the therapeutic agent is secondarily targeted by binding to one of the arms of the bispecific antibody that has localized at the site of the pathogen, the cancer or whatever lesion is being treated (Goldenberg et al., J Clin Oncol. 2006 Feb. 10; 24(5):823-34; Goldenberg et al., J Nucl Med. 2008 January; 49(1):158-63, each incorporated herein by reference in their entirety).